Capital and operational expenses of subsea developments, especially in deep waters, are high. Simple and reliable equipment is therefore important. Well maintenance costs are high due to the high intervention cost. Reliability of all this equipment is therefore a key word for success.
Flow assurance is of utmost importance for field economics. Water in the hydrocarbon stream is one of the frequent causes of flow related problems. Removing water will reduce possible hydrate formation and allow using flow lines with smaller diameter at reduced cost. Power needed for pressure boosting will be reduced due to the lower bulk flow and density.
Water is almost always present in the rock formation where hydrocarbons are found. The reservoir will normally produce an increasing portion of water with increase time. Water generates several problems for the oil and gas production process. It influences the specific gravity of the crude flow by dead weight. It transports the elements that generate scaling in the flow path. It forms the basis for hydrate formation, and it increases the capacity requirements for flowlines and topside separation units. Hence, if water could be removed from the well flow even before it reaches the wellhead, several problems can be avoided. Furthermore, oil and gas production can be enhanced and oil accumulation can be increased since increased lift can be obtained with removal of the produced water fraction.
A downhole hydrocyclone based separation system can be applied for both vertically and horizontally drilled wells, and may be installed in any position. Use of liquid-liquid (oil-water) cyclone separation is only appropriate with higher water-cuts (typical with water continuous wellfluid). Water suitable for re-injection to the reservoir can be provided by such a system. Cyclones are associated with purifying one phase only, which will be the water-phase in a downhole application. Using a multistage separation cyclone separation system, such as described in pending Norwegian patent application NO 2000 0816 of the same applicant will reduce water entrainment in the oil phase. However, pure oil will normally not be achieved by use of cyclones. Furthermore, energy is taken from the well fluid and is consumed for setting up a centrifugal field within the cyclones, thereby creating a pressure drop.
A downhole gravity separator is associated with a well specially designed for its application. A horizontal or a slightly deviated section of the well will provide sufficient retention time and a stratified flow regime, required for oil and water to separate due to density difference.
Separation of water from the hydrocarbon flow is therefore important. Such separation can be done at the seafloor and downhole. The separation process is however proven to be much more efficient downhole than at the seafloor. Such separation is also done more efficiently in each well bore than on the commingled well fluid from several wells. Downhole removal of water from the hydrocarbon flow, giving a less dense column, will result in a higher pressure available at the seabed. This will result in less need for pressure boosting for flow line transportation. Separation should therefore, if well conditions permit, rather be arranged downhole than subsea.
In copending Norwegian Patent Application No. 2000 1446 a system is described, in which a downhole turbine/pump hydraulic converter is used to inject the water into the formation to increase the pressure in the formation and thereby achieve more hydrocarbon output from the reservoir. This system is specially suitable for application in low to medium pressure wells, in which the water injection can increase the output.
However, in high pressure wells it is usually not of major benefit to inject water. Thus, a different system is needed for such wells. Since all rotating machinery (pumps and compressors) are among the most unreliable pieces of equipment of field developments, it is desirable to avoid such machinery downhole, where access and monitoring is difficult. In designing a system for exploitation of high pressure well it is therefor an object to avoid downhole rotating machinery as far as possible.
The alternative, locating the equipment topside, i.e. on the platform, is, as mentioned above, not a very good solution either. This calls for a subsea location of at least a part of such equipment.
However, downhole separation has major benefits over topside or subsea separation. This is due to the fact that the pressure gradient of hydrocarbons is steeper than the pressure gradient of the water. Downhole separation of the reservoir fluid thus gives a higher pressure of the hydrocarbons at the seabed than the total reservior fluid. A higher pressure means that the hydrocarbons can be transported over a further distance without additional pressure boosting or with less pressure boosting, than in the case of separation at the seabed or topside.